1. Field of the Invention
The invention relates to metal founding. More specifically, the invention relates to a method and apparatus for metering liquid metal during metal founding.
2. Description of the Related Art
Metering gates with three plates are used to control the rate of liquid metal flow exiting a teeming vessel, such as a tundish. For example, a metering gate may be used to control the rate of liquid steel flowing from the tundish of a continuous casting machine into a mold.
A metering gate consists of an assembly of refractory components, each of which has a flow channel. The flow channels (i.e. the holes or bores) within the refractory components are assembled together so as to provide a complete flow channel through the gate, which is in fluid communication with the teeming vessel and through which the liquid metal may be allowed to flow.
The refractory components of the metering gate are assembled and clamped together by mechanical means such that one component, a throttle plate, can slide laterally in the metering gate assembly to control the rate of liquid metal flow through the gate. By sliding the throttle plate to various positions, the gate may be either closed, partially open, or fully open to control the rate of flow exiting the teeming vessel.
Several problems are typically associated with controlling the flow of liquid steel exiting a tundish with metering gates. These problems include: (1) bending of metal flow in the flow channels of the gate, which can cause excessive turbulence and asymmetrical discharge of liquid metal; (2) severe non-uniform plugging of the flow channels from the accumulation of metallic and non-metallic materials which adhere to the channel walls with a subsequent loss of ability to obtain the desired rate and smoothness of liquid metal discharge; and (3) localized and accelerated eroding of a refractory component of the metering gate with subsequent contaminating of the liquid metal and potential loss of control or metal leakage.
Referring to FIGS. 1 and 2, a three-plate metering gate assembly 10 (hereinafter xe2x80x9cgate 10xe2x80x9d) typically consists of five basic components: a well nozzle 20, a top plate 30, a throttle plate 40, a bottom plate 50 and an outlet tube 60. Liquid metal (not shown) flows into gate 10 at the top and flows out of gate 10 at the bottom.
The well nozzle 20 is a pipe, which allows the entry of liquid metal flowing from the teeming vessel (not shown) into a flow channel bore 22 at the top of the well nozzle 20. The top plate 30 is in contact with the bottom of well nozzle 20, and includes a flow channel bore 32. The central axis 35 of the flow channel bore 32 in top plate 30, as shown in FIG. 2, is collinear with central axis 25 of flow channel bore 22 in well nozzle 20.
Throttle plate 40 is in contact with the bottom of top plate 30. Gate 10 is designed so that throttle plate 40 may slide laterally relative to the other components of gate 10. Bottom plate 50 is in contact with the bottom of throttle plate 40, and includes a flow channel bore 52. Central axis 55 of flow channel bore 52 in bottom plate 50 is collinear with central axis 25 of flow channel bore 22 in well nozzle 20.
Outlet tube 60 is in contact with the bottom of bottom plate 50, and includes a flow channel bore 62. Central axis 65 of flow channel bore 62 in outlet tube 60 is collinear with central axis 25 of flow channel bore 22 in well nozzle 20.
Central axes 25, 35, 55 and 65 of flow channels 22, 32, 52 and 62 in well nozzle 20, top plate 30, bottom plate 50 and outlet tube 60, respectively, are collinear and all together define the xe2x80x9cmain central axisxe2x80x9d 15 of gate 10.
As shown in FIGS. 3-5, throttle plate 40 slides between fully open (FIG. 3), partially open (FIG. 4) and gate closed (FIG. 5) positions. As shown in FIG. 4, during normal operations, throttle plate 40 typically is placed in a partially open position so that the flow rate of liquid metal through gate 10 may be metered, i.e., set and controlled, at a desired rate. As shown in FIG. 3, throttle plate 40 assumes a fully open position to maximize the flow of liquid metal through gate 10. As shown in FIG. 5, throttle plate 40 may assume a closed position, which would stop the flow of liquid metal through gate 10.
Metering gate components may be combined or subdivided. For example, to reduce the number of components, a gate 710 may be composed of only three parts, as shown in FIG. 6, in which the well nozzle may be combined with the top plate, defining a first component 712, and/or the bottom plate may be combined with the outlet tube, defining a second component 714, selectively placed in fluid communication with a throttle plate 740. As shown in FIG. 7, to more easily replace the outlet tube of a gate 810 having a well nozzle 812, a throttle plate 813 and a bottom plate 814, the bottom plate 814 may be divided into two plates 816 and 818.
Several variations of the fundamental three-plate gate components are used. For example, unlike the gate shown in FIGS. 1-5, in which well nozzle 20 has a tapered conical section bore 22 and bores 32 and 52 in plates 30 and 50 and bore 62 of outlet tube 60 define simple cylinders, as shown in FIG. 8, a gate 110 may have a well nozzle 120 with a cylindrical bore 122 and a top plate 130 with a conical bore section 132 with the bores in the throttle plate 140, the bottom plate 150 and the outlet tube 160 being the same as in the gate 110 of FIGS. 1-5. Also, as shown in FIG. 9, a gate 210 may have conical bore sections 222 and 232 in both well nozzle 220 and top plate 230 with the bores in the throttle plate 240, the bottom plate 250 and the outlet tube 260 being the same as in the gate 110 of FIGS. 1-5, and, as shown in FIG. 10, a gate 310 may have a well nozzle 320 having parabolically-shaped bore 322 and a top plate 330 having a conically-shaped bore 332 with the bores in the throttle plate 340, the bottom plate 350 and the outlet tube 360 being the same as in the gate 110 of FIGS. 1-5.
FIG. 11 illustrates another variation of a gate 410 where cylindrical bore 442 in throttle plate 440 is canted at an angle to plate surface 443 in an attempt to direct the flow through throttle plate 440 back toward main central axis 415 of gate 410. FIGS. 12 and 13 illustrate partially open and gate closed positions, respectively, of gate 410.
In gate 410, bores 422, 432, 442, 452 and 462 in well nozzle 420, top plate 430, throttle plate 440, bottom plate 450, and outlet tube 460, respectively, generally are axisymmetrical. For example, the bores have either cylindrical or conical section geometry. The central axis 425, 435, 455 and 465 of well nozzle 420, top plate 430, bottom plate 450, and outlet tube 460 generally are collinear.
Other variations of metering gates have been developed to provide for better draining of the throttle plate when it is closed. For example, FIGS. 14-16 show a gate 510, including a well nozzle 520, a top plate 530, throttle plate 540, bottom plate 550, and outlet tube 560, in open, partially open and closed gate positions, respectively. Gate 510 is similar to that of FIGS. 1-5 except that throttle plate flow channel bore 542 is extended by a special drain cut 544 near bottom edge 546 on one side to allow draining of bore 542 when the gate is in the closed position, as shown in FIG. 16. This prevents trapping of liquid metal in throttle plate bore 542 which otherwise would solidify when the gate 510 is temporarily closed.
FIGS. 17-19 show another gate 610, including a well nozzle 620, a top plate 630, throttle plate 640, bottom plate 650, and outlet tube 660, in open, partially open and closed gate positions, respectively, which provides another drainage feature. A conical bore section 652, at the top of bottom plate 650, has a diameter at top surface 654 of bottom plate 650 that is larger than the diameter of bore 652 at bottom surface 656 of bottom plate 650.
Unfortunately, the foregoing gate designs all provide a tortuous liquid metal flow path when the gate is partially openxe2x80x94the normal operating position during liquid metal pouring. Metering gates are designed with a maximum flow rate, but are intended to operate at about 50% of that rate. This assures the desired gate control response and affords excess capacity, which occasionally may be required for high-production or large section casting. Thus, a partially open gate is typical during liquid metal pouring, because the size of the flow channel must be large enough to provide a sufficient opening to accommodate a maximum rate of flow of the casting, but typically a gate is operated at less than maximum flow. The required or desired amount of liquid metal flow through the nozzle typically varies during the casting operation and generally is significantly less than the maximum, ranging from 30% to 70% of the maximum most of the time. As a result, the bent and contorted flow path formed in these gates when partially open causes: (1) asymmetric discharge of the liquid metal; (2) excessive turbulence in the flow channel; (3) localized regions which can be subject to accelerated erosion of refractory material; (4) over-restriction of the flow; and (5) rapid build-up of clogging in critical locations of the flow channel. The net effect is to shorten the useful life of the gate components and increase operating cost.
The distorted flow generated by these gates when partially open is illustrated schematically in FIGS. 20 and 21 with gates 210 (FIG. 9) and 410 (FIGS. 11-13) respectively. In FIG. 20, flow 271 in flow channel 212 impacts upper ledge 248 of throttle plate 240 (at Region A) which bends this portion of flow 271 sharply toward the opening of bore 242. Flow 272, which is the remaining portion of the flow, is bent to a much lesser degree. This mainly one-sided bending of the flow causes a flow 273 to separate from the surface of throttle plate bore 242 below the top edge 248 thereof and to be redirected toward bore 242. A high velocity jet flow 274 formed in throttle plate bore 242 is tilted strongly away from main central axis 215 of flow channel 212. This tilted jet impinges upon one side of bore 252 in bottom plate 250 (Region B) and feeds fluid into recirculating flow 275 under the ledge formed by the plate 230. The severe bending and tilting of the flow described above produces an asymmetrical flow pattern in bottom plate 250 and outlet tube 260 with: (1) a high speed flow 276 confined to one side of flow channel 212; and (2) an extensive recirculating flow 277, including very turbulent portions 278 and 279 which occupy the major portion of flow channel 212.
This flow behavior is deficient because it leads to excessive pressure loss and promotes clogging and erosion. The strong bending and tilting of the flow and its impingement on the refractory material (e.g. at Regions A and B), over-restricts the flow and the discharge of liquid metal is more easily impeded by any build-up of clogging material. Recirculating flow 275 is fed with incoming fluid providing ideal conditions for the build-up of non-metallic clogging material in bore 242 of throttle plate 240, which is a critical problem for gate performance. The asymmetrical nature of the flow in the outlet tube 260, with a concentrated jet 277 on one side and turbulent recirculation 279 on the other side, causes: (1) asymmetrical discharge of liquid metal from outlet tube 260, which can detrimentally affect cast metal quality; and (2) non-uniform and rapid clogging of outlet tube 260. Impingement of the flow on the sides of bore 252, such as in Region B, also aggravates problems with localized refractory erosion.
Referring to FIG. 21, one attempt to direct the flow back toward main central axis 415 of gate 410 fails and even exacerbates problems related to the tortuous flow path and the asymmetrical nature of flow distribution when gate 410 is partially open. FIG. 21 shows the flow pattern related to gate 410 having a canted cylindrical bore 442 in throttle plate 440 and a conical section bore 452 in bottom plate 450. The flow pattern is similar to, but more asymmetrical than, the flow of FIG. 20. Specifically, canted-throttle-bore flow 471 is bent more sharply where it impacts above top ledge 446 of throttle plate 440 (Region A), while flow 472 is bent much less than flow 471. This is because, comparing FIGS. 20 and 21, with a canted cylindrical bore 442, the entry of bore 242 essentially is shifted rightwardly, effectively presenting a longer ledge 446 which urges the flow 471 more orthogonal relative to the main central axis 415 than flow 271 interacting with a smaller top ledge.
The canting of bore 442 in throttle plate 440 also promotes a larger region of separated flow 473, as compared to FIG. 20, on one side of bore 242 in throttle plate 240. High velocity flow 474 is tilted more severely away from main central axis 415 of gate 410 which impinges more directly on one side of bottom plate bore 452 (Region B). Increased direct impingement of the jet increases the proportion of recirculating flows 475 and 476 under top plate ledge 446 and increases the confinement of high speed flow 477 entering outlet tube 460 to one side of flow channel 462. Subsequently, there is an increase in the extent of turbulent flow 478, 479 and 480 on the other side of flow channel 462. Thus, discharge is over-restricted and flow asymmetry entering outlet tube 460 is more severe, promoting clogging and erosion.
Accordingly, metering gate designs which attempt to improve flow symmetry by angling or canting the flow channel in the throttle plate to direct the flow back toward the main central axis of the gate when the gate is partially open are deficient and can cause greater problems during operation.
The foregoing demonstrates a need for a metering gate that promotes a straight liquid metal flow path.
The invention provides a method and apparatus for metering flow including selectively passing fluid through a passage in a top plate, having an inlet and an outlet, wherein the inlet and the outlet are offset, then into a throttle plate.
The invention provides for a metering gate which promotes a straighter liquid metal flow path and a more symmetrical and less turbulent discharge, thereby reducing the potential for clogging and erosion of the gate components. The invention provides for a reduction in the extent of separated and turbulent flow regions when the gate is partially open. The invention provides for less erosive flow behavior. The invention provides for less restriction when partially open, thereby allowing easier passage of the liquid metal. The invention provides for fewer clogging problems by retarding the rate of build-up, reducing the extent of build-up and improving the uniformity of any build-up. The invention provides for improved uniformity of flow distribution in the outlet tube, thus improved metal flow behavior in a downstream vessel, such as a continuous casting mold. The invention provides for easier draining of the throttle plate without detrimental effect on flow behavior. The invention provides improved elements and arrangements thereof, for the purposes described, which are dependable and effective in accomplishing intended purposes of the invention.